Endemic and invasive weeds are important management concerns in
California due to their direct and indirect costs to agriculture, the
environment and society. Pimentel et al. (2005) estimated that weeds
cost U.S. crop producers and pasture managers over $30
billion in control-related expenses and reduced productivity. Although
specific data are not available for California's portion of these
losses, weed management costs for the state's 40 million acres of
crop and grazing lands, as well as the remaining 60 million acres of
land area, amount, undoubtedly, to several billion dollars annually. In
addition to the direct cost of weed control and lost agricultural
productivity, weeds also affect ecosystem quality and function, reduce
recreational access and degrade aesthetics in natural areas, change
wildland fire regimes and severity, and impede water flow through rivers
and canals, among other negative impacts.

A stone fruit orchard in Fresno County is dominated by
glyphosate-resistant horseweed. Reliance on one method of weed control
imposes selection pressure, which can lead to population shifts to
tolerant species or selection of resistant biotypes. [Graphic
omitted]

Although crop weeds are seldom considered as being
'invasive' in the traditional sense, novel biotypes can
develop, spread and subsequently occupy a greater proportion of crop
acreage than might normally be expected. For example, when a weed
population evolves resistance to an herbicide or any other control
measure, a 'routine' pest can become a new and serious
problem. The first case of an herbicide-resistant weed in California was
reported in 1981 by UC scientists (Holt et al. 1981); in recent years,
additional species have evolved resistance to various herbicide
chemistries (table 1) used in some of California's signature
cropping systems, including flooded rice, orchards and vineyards as well
as nearby noncrop areas.

TABLE 1: Important herbicide modes of action

How do weeds become resistant to herbicides?

Environmental factors and production practices influence species
composition at any location, a phenomenon known as selection pressure.
Under constant conditions, the weed community will become dominated by
species that thrive under those conditions. If this steady state is
upset by a change in management practices, a weed shift may occur,
resulting in a community dominated by different species adapted to the
new conditions (Hanson et al. 2013). This weed shift can be caused by
agronomic and horticultural practices (tillage, fertility, irrigation,
etc.) or by the use of herbicides, which are very strong selective
agents. Some species will be less susceptible (more tolerant) than
others to any management practice, and repeated use of the same control
strategy can shift weed populations to become dominated by naturally
tolerant species (fig. 1A).

Herbicide resistance, on the other hand, implies that a genetic
change has caused a formerly susceptible population of a species to
become resistant to an herbicide. Herbicide resistance arises from the
process of adaptive evolution, whereby mutations change the physiology
of plants in such a way that the herbicide is less effective. Under the
continued selection pressure exerted by the herbicide(s), resistant
plants with the new genotype are not controlled, and their offspring
build up in the population (fig. 1B). Depending on the initial frequency
and genetic basis of resistance, the regularity and rate of herbicide
applications, and the reproductive system of the weed, it may take from
a few to many generations for resistance to become apparent (Jasieniuk
et al. 1996; Maxwell et al. 1990).

Fig. 1: Herbicides impose selection pressure and can lead to weed
species shifts, resulting in populations dominated by more-tolerant
species (A). Occasionally, an individual weed has a mutation that
confers resistance to an herbicide or group of herbicides, and this
individual survives and reproduces despite being treated with herbicide
(B). In both cases, after several generations and repeated selection
with the same or similar herbicides, the tolerant species or resistant
biotype can become dominant in the population. (Modified from Orloff et
al. 2009 with permission.) [Figure omitted]

Glyphosate-resistant horseweed in a raisin vineyard near Parlier,
left, and glyphosate-resistant ryegrass in a walnut orchard near Davis.

Current status of herbicide resistance

The strongest selection pressure for herbicide-resistant weeds tends
to be in modern, high-intensity agricultural cropping systems due to a
high reliance on herbicides. According to the International Survey of
Herbicide Resistant Weeds (weedscience.org), since the first confirmed
report of a resistant biotype in 1957, herbicide-resistant weed biotypes
have been reported in at least 60 countries and include more than 400
unique species-herbicide group combinations (fig. 2A). The United States
has more herbicide-resistant biotypes (162) than any other country (fig.
2B), and California accounts for 21 of these (fig. 2C, table 2).
Worldwide, resistance to acetolactate synthase (ALS)-inhibiting
herbicides and photosystem II (PSII)-inhibiting herbicides (Groups 5, 6
and 7) are the most commonly occurring among weedy species. However, in
recent years, glyphosate (glycine herbicide) resistance and multiple
resistances (resistance to two or more herbicides with dissimilar modes
of action) have also emerged as major problems in some cropping systems.
Interestingly, while herbicide resistance in the United States as a
whole is primarily found in broadleaf weeds, California has more
herbicide-resistant grasses or sedges (15) than broadleaf species (6)
(table 2).

Fig. 2: Chronological increase in reports of herbicide-resistant
weeds (HRW) worldwide and in the United States and California. Data
compiled in August 2013 from the International Survey of Herbicide
Resistant Weeds (weedscience.org).

TABLE 2: Confirmed cases of herbicide-resistant weeds in California

Due to the extensive use of preplant and in-season tillage in some
agronomic crops in California, along with the use of pre- and
postemergence herbicides, herbicide resistance is not as widespread as
it is in other parts of the country where no-till and minimum-till
systems have been widely adopted. Reduced tillage systems are heavily
reliant on a few herbicide modes of action (e.g., glyphosate) and have
correspondingly larger problems with herbicide resistance (Culpepper
2006).

In contrast to the rest of the United States, where herbicide
resistance problems are centered on agronomic crops, the greatest
problems with herbicide-resistant weeds in California are in orchards,
vineyards, flooded rice, roadsides and irrigation canal banks.
Herbicide-resistant weeds have become especially challenging problems in
California's signature cropping systems, which are characterized by
little or no crop rotation due to soil limitations (rice) or long
cropping cycles (orchards and vineyards) and relatively few
opportunities for mechanical weed control. Although large by specialty
crop standards, the approximately 3 million acres devoted to orchard,
vineyard and rice production in California is a small market for
herbicide manufacturers; thus, herbicide options are somewhat limited.
Combined, these factors have led to a high degree of selection pressure
for herbicide-resistant weed biotypes as well as weed population shifts
to naturally tolerant species (Hanson et al. 2013; Prather et al.
2000).

UC weed scientists address herbicide resistance in weeds

In order to combat complex issues such as herbicide resistance,
organized collaborations between weed scientists and other agricultural
researchers with a wide array of expertise are required. This includes
the activities of UC Cooperative Extension farm advisors and
specialists, Agricultural Experiment Station faculty, support
scientists, research staff and graduate students, as well as faculty
from other universities and agricultural industry representatives (for a
list of UC weed scientists, visit the Weed Research and Information
Center at wric.ucdavis.edu). Current herbicide-resistant weed management
efforts range from applied research and extension efforts to basic plant
biology and evolutionary ecology studies. Although the specifics vary,
these efforts can be grouped into three general areas: (1) applied
management of herbicide-resistant plants, (2) physiology and mechanisms
of resistance and (3) biology, ecology and evolution of herbicide
resistance.

Applied management of herbicide-resistant plants.

Many cases of herbicide resistance in weeds are identified after
growers, land managers or pest control advisers observe weed control
failures with treatments that were once effective. These weeds are
generally brought to the attention of local or statewide Cooperative
Extension personnel. If the herbicide application method is ruled out as
the cause of poor weed control (i.e., incorrect product, rate, timing,
placement, etc.), researchers often conduct field or greenhouse tests to
verify and quantify the level of resistance. Plants from the suspected
herbicide-resistant population are treated with the herbicide of
interest at rates ranging from below normal doses to doses well above
those legally allowed in the field (see photos, below). The response
(i.e., plant growth or mortality) of the putative resistant population
is then compared with the response of the known susceptible, or
wild-type, population. Resistance is confirmed if the herbicide affects
the two (or more) populations of the same species in markedly different
ways with respect to plant growth and survival. In many cases, an
estimate of the level of resistance also is made from these data. For
example, if the susceptible population is controlled at one-half the
field rate, but the resistant population survives at twice the field
rate, it would be described as having a fourfold (2 / 0.5 = 4) level of
resistance.

Orchard-collected junglerice plants 21 days after treatment in a
greenhouse dose-response experiment. The pot at the farthest left in
each photo was untreated, and the remaining plants were treated with
glyphosate rates ranging from (left to right) 1/32x, 1/16x, 1/8x, 1/4x,
1/2x, 1x, 2x and 4x of the labeled use rate. The glyphosate-susceptible
population was controlled with a 1/4 use rate, while the resistant
population had some survivors at the 4x rate.

Physiology and mechanisms of herbicide resistance.

Identifying and verifying herbicide resistance and developing
alternative management strategies provides short-term solutions for weed
managers. Researchers often conduct further studies to determine the
underlying molecular and physiological causes of resistance and to
compare the biology, growth and competitive ability of
herbicide-resistant species and biotypes. The mechanism(s) and fitness
costs of herbicide resistance can have important ramifications on the
selection, spread and competitive ability of herbicide-resistant
biotypes, in addition to directly impacting their management. The goal
of these efforts is to help growers and pest control advisers recognize
the importance of taking a proactive approach to preventing the
evolution of a resistant population, rather than a reactive approach to
managing herbicide resistance after it occurs.

Target-site resistance occurs when the enzyme that is the target of
the herbicide becomes less sensitive, or fully insensitive, to the
herbicide, often due to a physical change in the target enzyme's
structure. These physical changes can impair the ability of the
herbicide (or other herbicides) to attach to a specific binding site on
the enzyme, thus reducing or eliminating herbicidal activity.
Target-site resistance is sometimes evaluated at the tissue level using
portions of plants such as leaves, leaf disks or roots (see photos
below). In some cases, a functioning target enzyme (e.g., ALS or acetyl
coenzyme A carboxylase [ACCase]) can be extracted and its function
evaluated in laboratory in vitro experiments in the presence or absence
of the herbicide. Recently, overproduction or enhanced activity of the
target enzyme has been shown to confer herbicide resistance in certain
cases (Gaines et al. 2011).

In some cases of herbicide-resistant weeds, enzyme- or tissue-level
assays can be used to understand and quantify resistance. Above, a lab
assistant collects leaves from suspected glyphosate-resistant horseweed;
left, leaf disks from the intact leaves are cut for an in vivo assay;
right, disks are incubated overnight in the laboratory in buffer
solutions containing various concentrations of glyphosate in order to
evaluate activity of the EPSPS enzyme.

Several mechanisms of nontarget-site resistance confer resistance to
herbicides in plants without involving the target sites of the
herbicides. This can result in unpredictable resistance to unrelated
herbicides (Délye 2013; Délye et al 2013). Of
these, the best-known cases involve resistance in which
herbicide-resistant plants have an enhanced ability to metabolically
degrade the herbicide to less- or nontoxic forms. Many processes can be
involved in metabolic resistance, but the most well-understood cases are
due to changes in three groups of isozymes (cytochrome P450 monoxidases,
glutathione transferases and glycosyltransferases) and changes in
ATP-binding cassette (ABC) transporters (Yuan et al. 2007). This type of
resistance is most commonly evaluated using nonherbicidal inhibitors of
the various isozymes in the presence or absence of the herbicide and
comparing metabolic degradation of the herbicide in laboratory or
greenhouse assays.

Biology, ecology and evolution of herbicide resistance.

Many factors influence the evolution of herbicide resistance in weed
populations (reviewed in Jasieniuk et al. 1996). To design effective
resistance management strategies for the long term, UC and other
scientists are conducting basic research on weed biology and on
ecological and evolutionary processes in weed populations.

In a few cases, the mechanisms that confer resistance to herbicides
have altered the fitness (i.e., survival, growth and/or seed production)
of resistant plants, as compared with susceptible plants of the same
species in the absence of herbicide treatment. Differential plant
fitness among biotypes can affect the rate at which herbicide resistance
can spread. For example, if resistant and susceptible plants have equal
fitness, the number of resistant plants in the population would not
change relative to the number of susceptible plants during periods when
the herbicide was not being applied (Jasieniuk et al. 1996). In
contrast, if resistant plants are less fit than susceptible plants, the
number of resistant plants may decrease during periods when herbicide is
not applied. Fitness is usually evaluated by growing resistant and
susceptible plants in direct competition with one another, or with the
crop of interest, and comparing relative productivity or fecundity.

Similar to efforts for other invasive weeds, insects and disease
pathogens, surveys are sometimes used to delineate the extent of
population growth or the expansion of new herbicide-resistant weed
biotypes. Because there often are a few escaped weeds in
herbicide-treated fields, herbicide resistance may not be recognized
until the resistant biotype makes up a significant portion of the local
population (Vencill et al. 2012). Surveys can help inform growers of
emerging herbicide-resistant weed populations while they are still
localized; surveys are also often used to encourage adoption of
resistance mitigation measures to minimize economic and environmental
impacts. Further, surveys combined with population genetic research can
determine the evolutionary and geographic origins, and routes of spread,
of resistance across an agricultural landscape (e.g., Okada et al. 2013;
Okada et al. 2014).

Herbicide resistance in California

Herbicide resistance has been an important management concern in
California flooded rice production for several years (Busi et al. 2006).
Weeds with resistance to the ALS inhibitors (Group 2), thiocarbamates
(Group 8) and ACCase inhibitors (Group 1) are the dominant weed
management problems in most of the Sacramento Valley rice production
region. In orchards and vineyards, herbicide resistance is a more recent
development and is dominated by resistance to the broad-spectrum
postemergence herbicide glyphosate. This herbicide is, by far, the most
widely used herbicide in the state in perennial crop production systems,
as well as in many roadsides, canal banks and residential and industrial
areas. Glyphosate-tolerant (Roundup Ready) cotton, alfalfa and corn are
becoming widely adopted in the state, which will further increase
selection pressure for additional glyphosate-resistant and -tolerant
species.

Herbicide resistance in flooded rice.

Most California rice is produced in monoculture systems due to
impeded soil drainage, which limits rotation to other upland crops (Hill
et al. 2006). Rice fields are kept under continuous flood conditions
during the growing season, primarily for the control of grass weeds
(Strand 2013). Although this system favors sedges and other
water-tolerant weeds, selective herbicides such as molinate and
bensulfuron provided highly effective weed control for several years.
However, in the early 1990s, after repeated use, resistance to the
ALS-inhibiting herbicide bensulfuron became widespread among weedy
species in rice. By 2000, several additional weed biotypes with
resistance to ALS inhibitors, thiocarbamates or ACCase inhibitors had
evolved and were causing significant weed management, economic and
environmental issues in the rice cropping system. UC researchers,
extension personnel and industry partners have devoted considerable
efforts to understanding and managing herbicide-resistant weeds in
rice.

Smallflower umbrella sedge (Cyperus difformis) and California
arrowhead (Sagittaria montevidensis) resistance to ALS-inhibiting
herbicides was first reported in California rice fields in 1993
following repeated use of bensulfuron (Hill et al. 1994). Field research
has shown that California arrowhead is a fairly weak competitor in rice
systems (Gibson et al. 2001) and that the ALS-resistant biotypes can be
adequately controlled with other registered herbicides. Recently,
smallflower umbrella sedge biotypes with multiple resistance to the PSII
herbicide propanil and to several ALS-inhibiting herbicides were
identified in the Sacramento Valley (Valverde et al. 2014), and research
is ongoing to elucidate the mechanisms of resistance and any cross
resistance to other rice herbicides.

Eared redstem (Ammannia auriculata) and ricefield bulrush
(Schoenoplectus mucronatus) resistance to ALS inhibitor herbicides in
rice was reported in 1997. Redstem research has focused on intra- and
interspecific competition in an effort to develop agronomic solutions to
reduce its competition with rice (Caton et al. 1997; Gibson et al.
2003). Studies have shown that California populations of ricefield
bulrush are resistant to most registered ALS inhibitors, whereas
populations from other regions are resistant only to one chemical
family, the sulfonylureas, in the ALS inhibitor group (Busi et al.
2006). Recently, ricefield bulrush biotypes with multiple resistance to
propanil and bensulfuron were identified in the Sacramento Valley
(Abdallah et al. 2014).

Late watergrass (Echinochloa phyllopogon) populations resistant to
ACCase inhibitors, ALS inhibitors and the thiocarbamate herbicides in
rice systems were reported in 1998 (Fischer, Ateh et al. 2000; Fischer,
Bayer et al. 2000). This resistance to multiple herbicides within an
individual plant indicated that using herbicides with different modes of
action would be unlikely to provide satisfactory control of the species
in the long term. Further complicating the situation in rice,
populations of late watergrass and barnyardgrass (Echinochloa
crus-galli) with resistance to both ACCase inhibitors and
thiocarbamates, and thus exhibiting multiple resistance, were reported
in 2000. Later research confirmed that the mechanisms of multiple
resistance to several herbicide classes are due to metabolic degradation
of these compounds (Yasuor et al. 2008, 2009).

Barnyardgrass (Echinochloa crus-galli).

Smooth crabgrass (Digitaria ischaemum) resistance to the synthetic
auxin herbicide quinclorac was reported in 2002. Detailed research into
the mechanisms of resistance suggested that the cause was an altered
sensitivity in the auxin response pathway, leading to ACCase activity,
ethylene synthesis and enhanced ability to detoxify cyanide (a byproduct
of ethylene biosynthesis) (Abdallah et al. 2006). Although crabgrass is
not an important rice weed, quinclorac is used in rice systems for
control of other weeds, and resistance to it has been reported in
Echinochloa species of rice in California (Yasuor et al. 2011) and from
other regions. Most importantly, the observed changes in ethylene
synthesis and production of toxic byproducts may also relate to the
plant's ability to tolerate abiotic stress. Two implications of
this finding include the possibilities that (1) quinclorac-resistant
smooth crabgrass has the potential to invade a more diverse range of
habitats and become an important weed of rice; and (2) adaptation to the
abiotic stress of the flooded environments may predispose Echinochloa
phyllopogon or other major rice weeds to evolve resistance to quinclorac
in the future.

Herbicide resistance in orchard and vineyard cropping systems.

The first herbicide-resistant weed in orchard cropping systems was
perennial ryegrass, Lolium perenne (now named Festuca perennis spp.
perenne), reported in 1989 (Heap 2013). This ALS inhibitor-resistant
biotype was selected on roadsides by the use of sulfometuron and, thus
far, has not been a major problem in orchards or vineyards because
relatively little of this class of herbicides is used in these crops.
However, several ALS inhibitors, including rimsulfuron, penoxsulam,
halosulfuron and flazasulfuron, are becoming more widely used in tree
and vine crops, and selection pressure for ALS inhibitor resistance may
increase in the future.

The first case of glyphosate resistance in California was reported
in a population of rigid ryegrass (Lolium rigidum, now Festuca perennis
spp. rigidium) in 1998 (Simarmata and Penner 2008). However, most
confirmed glyphosate-resistant ryegrass populations have been identified
as Italian ryegrass (Lolium multiflorum, now Festuca perennis spp.
multiflorum) (Sherwood and Jasieniuk 2009). Glyphosate-resistant
ryegrasses have become widespread and are a major weed problem in
orchards, vineyards and roadsides of Northern California (Jasieniuk et
al. 2008). Research indicated that resistance in ryegrass is not due to
metabolism of the herbicide and is instead due to an altered EPSPS
enzyme (Jasieniuk et al. 2008; Simarmata and Penner 2008). Glyphosate
resistance in these areas has been largely driven by decreases in grower
use of other herbicides, especially those under increasing regulatory
pressure because of pesticide contamination of ground or surface water.
The use of glyphosate-based herbicide programs also increased when the
patent on Roundup expired in 2000 and low-cost, generic glyphosate
herbicides became readily available. Today, glyphosate accounts for over
60% of all herbicide-treated acreage in California orchard and vineyard
systems (DPR 2013).

Glyphosate-resistant horseweed, or mare's tail (Conyza
canadensis), was reported in 2005 and is one of the dominant weeds in
and around raisin and tree fruit production areas of the San Joaquin
Valley, as well as on roadsides and canal banks in the region (Hanson et
al. 2009; Hembree and Shrestha 2007; Shrestha, Hembree, Wright 2008;
Shrestha et al. 2010). The level of glyphosate resistance in horseweed
is relatively low, and resistant plants are usually injured to some
degree following glyphosate applications, which suggests that resistance
is not due to an altered target enzyme. Genetic comparisons of horseweed
accessions from around the state suggest that there have been multiple,
independent origins of resistance in this species, rather than the
spread of resistance from a single-source population (Okada et al.
2013).

Hairy fleabane (Conyza bonariensis) populations resistant to
glyphosate were reported in 2007 (Shrestha, Hanson, Hembree 2008).
Glyphosate resistance in hairy fleabane appears to be similar to
resistance in horseweed in that (1) selection has occurred in response
to similar management strategies in perennial crops and surrounding
areas (Hembree and Shrestha 2007); (2) multiple origins of resistance
are suspected (Okada et el. 2014); and (3) growth stage and
environmental conditions affect the level of resistance (Moretti, Hanson
et al. 2013; Shrestha et al. 2007). The discovery by Moretti, Hanson et
al. (2013) of hairy fleabane resistant to both glyphosate and paraquat
raises questions about whether a common physiological mechanism is
helping to confer resistance to these dissimilar herbicides, and
research is ongoing to elucidate these factors.

Junglerice (Echinochloa colona) resistant to glyphosate was first
identified in 2008 in a Roundup Ready corn field in the Sacramento
Valley (Alarcon-Reverte et al. 2013); since then, glyphosate-resistant
junglerice has become widespread in orchards and field crops throughout
California (Moretti, Garcia et al. 2013). Resistance appears to be due
to mutations in the EPSPS target site (Alarcon-Reverte et al. 2013),
although some populations also appear to have enhanced EPSPS activity
(A.J. Fischer, unpublished data). Target-site mutations appear to be the
most frequent mechanism among the accessions so far collected in
California; however, additional research is ongoing to determine whether
the same is true with populations selected in orchards and in other
regions of the Central Valley.

Several other common weeds in orchards and vineyards, including
Palmer amaranth (Amaranthus palmeri), three-spike goosegrass (Eleusine
tristachya) and witchgrass (Panicum capillare), are suspected to have
evolved resistance to glyphosate; preliminary research trials by UC
researchers and California State University, Fresno, collaborators have
been initiated to verify and characterize the putative resistant
populations.

California herbicide resistance research: Locally applied research
and extension with national and international implications

Since the discovery of herbicide-resistant weed biotypes in
California, UC research and Cooperative Extension personnel, as well as
university and nonuniversity cooperators and students, have conducted
locally relevant weed management research in the state. Research and
extension efforts have included alternative chemical management
techniques using various postemergence and preemergence herbicides along
with mechanical control measures in an integrated approach. Applied
research integrating agronomy, weed control, spray application
technology and water management have been useful to regulatory agencies
(e.g., California Department of Pesticide Regulation and California
Environmental Protection Agency) and have had positive impacts on water
and air quality, wildlife habitat and water use (Pittelkow et al.
2012).

Information on the underlying mechanisms and genetic basis of
resistance provides useful information to California weed managers in
the longer term. This information is broadly applicable to the biology,
physiology, evolution and control of weeds in other crops and regions at
the local, national and international level. Although this paper has
focused on the efforts of UC weed scientists and collaborators, the
basic and applied scientific information developed in California
supports similar research being conducted in other regions of the
country and world.

Like many other areas encompassed by the Endemic and Invasive Pests
and Diseases Strategic Initiative, solutions to herbicide resistance are
not simple and are affected by many biological, economic, regulatory and
social factors. The diverse network of weed scientists and collaborators
in a land-grant university system is well positioned to address these
complex issues and respond to stakeholder concerns through applied and
basic research, extension and outreach to affected agricultural
industries, and education of future scientists and leaders. Ultimately,
the goal of weed science research is to help growers maintain
agricultural productivity and economic stability while increasing
environmental sustainability.

Worldwide, the majority of the plant species that are developing
herbicide resistance are those that occur as weeds in agricultural
environments, on roadsides and in other rights-of-way. In contrast,
herbicide resistance is not nearly so common in weeds of natural areas
or rangelands. A search of the International Survey of Herbicide
Resistant Weeds (weedscience.com) revealed no herbicide-resistant weeds
(i.e., invasive nonnative species) listed for terrestrial natural areas
anywhere in the world, and only two resistant weeds listed for aquatic
areas, both of them in Florida. In pastures, 15 species worldwide have
developed resistance, eight of which are considered primarily as
agricultural weeds. Only two of those 15 are found in pastures within
the United States, and none occurs in any Western state.

The reason more weeds develop herbicide resistance in agricultural
and right-of-way systems has to do with factors associated with
characteristics of specific weeds, herbicides and weed management
practices. For example, high seed production increases the opportunity
for genetic variation, and with it the probability that a resistance
adaptation will occur. It so happens that all of the major weeds that
have developed resistance to herbicides are annuals. In an agricultural
system, annual species make up the vast majority of problematic weeds.
Annuals can have high seed production, rapid turnover of the seedbank
(due to a high percentage of seed germination each year) and, in some
cases, several reproductive generations per growing season. This
increases the selective pressure for herbicide-resistant biotypes. In
natural areas of California, the California Invasive Plant Council
(Cal-IPC) lists 214 flowering plants as invasive (cal-ipc.org). Of
these, only 27.5% are annual species; the remainder (and the majority)
are either woody species or herbaceous perennials or biennials.
Perennial weeds, and particularly those with vegetative reproductive
tissues, are less likely than annuals to evolve herbicide
resistance.

The choice of herbicide can also increase or decrease the likelihood
that weeds will develop herbicide resistance. In most natural areas,
herbicides are not used as intensively as in croplands, where it is
common to repeat herbicide applications within a single year or over
several consecutive years. In addition, fewer herbicides are available
for use in natural areas of California, and the most widely used
compounds (e.g., 2,4-D, aminopyralid, dicamba, triclopyr or clopyralid)
belong to the growth regulator chemical families. Resistance to these
herbicides does not develop as commonly as resistance to other herbicide
families, despite their having been available and extensively used for a
long time. Glyphosate is also commonly used in natural areas, and
although glyphosate resistance is on the rise in cropping systems, its
development is often associated with repeated applications over multiple
years, a strategy not generally used in natural areas.

Weed management practices are often the most important contributing
factors leading to the selection of herbicide-resistant biotypes. In
general, a land manager's complete and repeated reliance on a
single herbicide or mode of action for weed control can greatly enhance
the occurrence of herbicide-resistant weeds. This is particularly true
when the manager uses no other weed control option, such as mechanical
or cultural control practices. For a number of reasons, including
economic feasibility and the potential for damage to desirable
(nontarget) vegetation, it is uncommon for a land manager to reapply the
same herbicide for several consecutive years in a natural area.

Because the evolution of herbicide resistance is typically the
result of intensive, persistent selective pressure on a rapidly
regenerating weed population (i.e., annual species), the incidence of
herbicide-resistant species would be expected to be much higher in a
cropping system with limited rotations or in other systems, such as
rights-of-way, that are continuously managed with herbicides. In many
natural areas, the effort to manage invasive plants can involve several
different control strategies besides, or instead of, herbicide
application. These can include mechanical means such as mowing, cultural
methods including grazing management or prescribed burning and, when
available, biological control agents. Furthermore, even when herbicides
are used, they are rarely applied repeatedly over a long period of time.
The total area of noncropped lands treated with herbicides is far
smaller than the total acreage of agricultural land treated with
herbicides. It is hardly surprising, then, that the incidence of
herbicide resistance in natural areas and rangelands is low -- in fact,
it is not even reported at present in California.

Regardless of the vegetative environment, whether natural or
agricultural, prevention of herbicide resistance and management of
established resistant weed populations could be accomplished more
effectively if we put a greater reliance on integrated weed management
approaches. Although the likelihood that resistance will develop in
natural areas is already low, management strategies that employ rotation
of herbicides with different modes of action, the use of competitive
species in restoration programs, and a combination of mechanical,
biological and cultural control options in an integrated management
program will further reduce the selective pressure on invasive plant
populations and with it the potential that weeds will develop herbicide
resistance.

Weed management systems in California vegetable crops can be
described as robust, complex, multitactic and integrated. Vegetable
herbicides generally make up just one component in a multicomponent weed
management system. With California's seasonally dry weather and
growers' ability to control soil moisture by means of irrigation
scheduling, it becomes possible for the grower to apply effective
cultural and physical control practices, such as preparation of stale
seedbeds and inter-row cultivation. Redundancy is designed into the weed
management system to minimize weed emergence in the crop. The key tools
that make up an integrated vegetable weed management system are field
selection, sanitation, crop rotation, land preparation, stale seedbeds,
herbicides and physical weed control (UC IPM 2009). Growers who
carefully apply these practices are able to manage weeds effectively and
reduce the presence of weed seeds in the soil seedbank.

Field selection.

Farmers often grow vegetable crops on fields that have low weed
pressure so their weed control operations can be more efficient and
economical. They use translocated herbicides during fallow periods to
control troublesome perennial weeds like field bindweed.

Sanitation.

Growers often keep vegetable fields and the surrounding areas as
weed-free as possible to keep the weeds from going to seed. Some
operations that utilize a 'zero weed seed' philosophy have
successfully reduced weed pressure in subsequent vegetable crops by
eliminating weed seed inputs to the soil seedbank. Other measures such
as cleaning all field equipment when moving it from a weedy field or
into a clean field are also employed.

Rotation.

By growing vegetable crops in rotation with crops that normally have
more intensive weed control programs, growers can help keep a field
clean of weeds. Because field conditions are constantly changing under a
rotation system, no one weed species is likely to become dominant.

Land preparation.

Direct-seeded vegetable crops require well-prepared seedbeds free of
large clods to facilitate precision planting and allow rapid and uniform
emergence of vegetable seedlings. A uniform seeding depth is critical to
uniform crop emergence and improved tolerance to herbicides. Mechanical
cultivation is facilitated with smooth seedbeds and good tilth, which
allows the cultivation equipment to remove weeds that are close to the
crop row. Increasingly, growers are using precision guidance systems to
improve the speed and accuracy of cultivation.

Preirrigation and use of a stale seedbed.

Preirrigation before final seedbed preparation is a common practice,
as it stimulates a weed flush a few days after watering. As soon as the
weeds have emerged and the field is dry enough to enter, the grower uses
shallow cultivation, flaming or a nonselective herbicide to remove the
new weeds. Research has shown this technique to provide 15% to 50%
control of weeds in crops like lettuce (Shem Tov et al. 2006). The
combination of stale seedbed technique and both herbicides and
cultivation often results in good weed control.

Herbicides.

One category of herbicide used in vegetable crops is fumigants, such
as metam potassium, which is applied 14 to 21 days before planting to
kill weed seeds and germinating seedlings. After planting, soil-active
herbicides like pronamide (used in artichokes and head lettuce) and
S-metolachlor and trifluralin (used in tomatoes and peppers) are applied
to provide preemergence control of weeds. Postemergence herbicides are
utilized in some crops; examples include clethodim, used to control
emerged grass weeds in many broadleaf vegetable crops, and oxyfluorfen
and bromoxynil, used to control broadleaf weeds. Many vegetable
herbicides were developed in the 1960s and 1970s and include products
like DCPA (used in broccoli and onion), napropamide (used in tomatoes
and peppers) and linuron (used in asparagus and celery). Given the
complexity of the vegetable weed control program and the extensive use
of cultivation and hand-weeding, the selective pressure on weeds from
vegetable herbicides is very light, despite their decades of use.

Physical weed control.

Vegetable growers make extensive use of physical weed control. One
example is inter-row cultivation or shallow cultivation between the crop
rows to control weeds. Inter-row cultivation is a very old but effective
method that buries, cuts or uproots weeds. Hand-weeding by workers with
hoes is the last line of defense against weeds in vegetable crops. Among
the hoeing crew, manual dexterity and good depth perception allow the
workers to carefully remove weeds from the vegetable crop in the row and
near the crop plant. Hand-weeding is expensive and can cost
$300 or more per acre in organic vegetable plantings and
high-density plantings (e.g., spinach and baby lettuces) -- sometimes a
lot more.

Integrated weed management in lettuce.

In a typical lettuce weed control program, the crop is grown on a
field with a light weed population, so one tool growers use is field
selection. Sometimes the soil is fumigated with metam potassium before
planting to control weeds and soilborne diseases, but most lettuce is
grown on nonfumigated land. Prior to planting, the soil is irrigated to
stimulate weed emergence and then shallow-tilled to kill weeds and form
a smooth seedbed for planting. At time of seeding, preemergence
herbicides such as pronamide or bensulide are applied, to be activated
with the initial sprinkler irrigation. About 4 weeks after emergence,
the lettuce is hand-thinned and weeded by a hoeing crew to its final
stand. Inter-row cultivation in furrows and on bed tops is conducted one
or more times, also removing weeds. Finally, about 6 weeks after lettuce
emergence, the field is hand-weeded to remove any remaining weeds. After
harvest, the field is quickly tilled under, killing any remaining weeds
before the field is rotated to another crop.

Integrated weed management in tomatoes.

Virtually all California tomatoes are transplanted, and 75% are
grown using subsurface drip irrigation buried 8 to 10 inches deep.
Fields with low weed populations, especially those free of field
bindweed and dodder, are most often sought for tomato production. Beds
are preirrigated to germinate weeds, then cultivated and shaped prior to
planting. Typically only two herbicide applications are made: one just
prior to planting or at planting, and the other at layby. Herbicides
such as halosulfuron, pendimethalin, rimsulfuron, S-metolachlor,
sulfentrazone and trifluralin are used, depending upon the site and weed
spectrum. Just prior to layby application, beds and furrows are
mechanically cultivated.

These practices significantly reduce weed emergence and competition
against the young tomato crop. Hoeing crews may hand-weed once or twice
before or after layby, depending on weed populations. Adding to the cost
for growers who practice 'zero weed seed tolerance' is the
physical removal of troublesome weeds such as flowering nightshades and
dodder to prevent seeding and further field contamination. The harvest
operation undercuts all plants growing on the bed top, and after harvest
the field is quickly tilled under, killing any remaining weeds before
the field is rotated to another crop.

The lettuce and tomato weed management systems are intensive and
redundant, made up of many operations conducted in sequence with the aim
of minimizing weed emergence. In practice, these weed management systems
are not always as flawless as the above descriptions might suggest.
Crops like broccoli and cauliflower are grown during winter months, when
extended rain and wet field conditions prevent cultivation and
hand-weeding. Other complications are high-density plantings such as
those used for spinach, which limit the grower's ability to
cultivate.

Overall, the chances are low that weeds will develop herbicide
resistance in a vegetable crop planting. Technology is evolving that
will allow intelligent robotic cultivators to remove weeds from the
intra-row space without the use of herbicides, so there is reason for
optimism that the development of herbicide-resistant weeds in California
vegetable fields will remain low for the foreseeable future.

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